/Engine/Source/Runtime/Core/Public/Containers/UnrealString.h
CharType is not const to use this overload for mutable char arrays and call Strlen() instead of getting the static length N from GetNum((&T)[N]). Oddly MSVC ranks a const T* overload over T&& for T[N] while clang picks T&&.
template<class CharType> [FString](API\Runtime\Core\Containers\FString) & Append ( CharType * Str )